The present invention relates to a probe for a nuclear magnetic resonance apparatus (hereinafter called an NMR apparatus), and more particularly to a probe for NMR measurement that has the circuit configuration and the installation structure of a radio frequency (RF) coil used for transmitting a high-frequency signal at a predetermined resonant frequency to a sample placed in an evenly distributed magnetic field and/or receiving a free induction decay signal (FID signal).
Nuclear magnetic resonance (NMR) spectral measurement, which gives information on a substance at an atomic level, is an excellent measurement method for investigating the structure of a compound. The basic principle of the measurement operation is that a high-frequency magnetic field is applied to a sample placed in an evenly distributed static magnetic field and the response signal from the excited nuclear spin is received and analyzed. The NMR apparatus, capable of analyzing the structure of a compound at an atomic level and measuring from a solid sample to a liquid solution, is used also for analyzing the structure and the function of proteins, structural analysis of a low-molecular compound, etc.
An analysis by means of an NMR apparatus sometimes requires that signals from multiple nuclides be measured at the same time. A probe used for such measurement must have the function to send and receive signals at the frequencies corresponding to multiple nuclides at the same time. For example, an NMR probe for measuring protein substances generally has a radio frequency (RF) coil or a radio frequency (RF) coil circuit installed for simultaneously measuring the four types of nuclides, H, D, C, and N. In such a multiple nuclei measuring probe, one radio frequency (RF) coil is generally resonated by multiple frequencies (multiple resonance). That is, the resonance characteristics of the radio frequency (RF) coil are tuned with the resonant frequency of the nuclides and, at the same time, the impedance at each resonant frequency is matched to a particular value (generally, 50Ω). An example of the configuration of a double resonant circuit that makes possible a multiple nuclei measurement is disclosed in Patent Document 1 (JP-A-2003-302452) and Patent Document 2 (JP-A-2005-140651).
The sensitivity of NMR measurement depends on the Q value of the radio frequency (RF) coil circuit. The Q value is a factor that depends on the resistive loss in the radio frequency (RF) coil circuit; the lower the resistive loss is, the higher the Q value is and the higher the detection sensitivity is. The conventional radio frequency (RF) coil circuit disclosed in the Patent Document 1 and Patent Document 2 is constructed based on the circuit shown in FIG. 1. In this circuit configuration, the capacitance of matching capacitors 44 and 46 is high with the problem that the resistive loss in the circuit is increased. In the case of the material and the shape of a general radio frequency (RF) coil, the capacitance value is in the range from several tens of pF to 100 pF. The problem with a chip capacitor normally used in a radio frequency (RF) coil circuit is that, when the capacitance is increased, the resistances 43 and 47 due to an inductive loss are increased because of the characteristics of the parts. So, when such a high-capacitance part is used, the Q value is decreased because of an increase in the resistive loss in the whole radio frequency (RF) coil circuit and therefore the detection efficiency is decreased.
This problem becomes obvious when the resistance of the RF coil is decreased. To increase the Q value of a radio frequency (RF) coil, it is effective to cool the RF coil for decreasing the resistance or to use a low-resistance material such as a superconducting material. However, when the resistance of the RF coil is decreased, it is necessary to further increase the value of the matching capacitor 44 for impedance matching. So, even if the RF coil has a high Q value (low resistive loss), the problem is that the Q value of the whole circuit cannot be sufficiently increased because the loss of the capacitor cannot be decreased.
[Patent Document 1] JP-A-2003-302452
[Patent Document 2] JP-A-2005-140651